The present invention relates to a bipolar electrolysis apparatus with an oxygen-consuming cathode for the production of chlorine and caustic soda from aqueous alkali metal chloride solution, with devices for supplying the electrolysis current and the electrolysis feed materials and for discharging the electrolysis output products, the anode and cathode being arranged to be separated from one another by means of a partition.
The electrolysis of aqueous sodium chloride is an important process for the production of the heavy chemicals chlorine and caustic soda. A modern variant is carried out in a membrane cell. In this process, the electrolysis cell consists of an anode space with an anode and a cathode space with a cathode, and of a cation exchanger membrane which separates the two electrolysis spaces from one another. When a saturated sodium chloride solution is fed into the anode space, the chloride ions are discharged at the anode to elemental chlorine under the action of the electric current. At the same time, a decomposition of water with the formation of elemental hydrogen and hydroxide ions takes place at the cathode. Approximately at the same rate as that of the generation of hydroxide ions, sodium ions migrate from the anode space through the cation exchanger membrane into the cathode space. The underlying chemical reaction corresponds to the following equation: EQU 2 NaCl+2 H.sub.2 O.fwdarw.2 NaOH+Cl.sub.2 +H.sub.2
For the anode space of an electrolysis cell, in which an alkali metal chloride, such as, for example, sodium chloride, potassium chloride or lithium chloride, is to be electrolyzed, a material must be used which is resistant to the corrosive medium which contains high chloride ion concentrations and elemental chlorine. In the state of the art, titanium, iridium or precious metals are used, and titanium metal is preferred which can have been superficially activated with a mixed oxide in order to reduce the chlorine overvoltage and at the same time to increase the oxygen overvoltage. The anode likewise consists of titanium, which can have been activated by transition metal oxides, such as ruthenium oxide or iridium oxide, in order to lower the chlorine overvoltage and at the same time to increase the oxygen overvoltage.
Titanium cannot be used as the material for the cathode space, since the hydrogen formed would cause an embrittlement of the titanium metal. The cathode space is therefore made of ordinary steel, stainless steel, nickel or nickel-plated steel. The cathode likewise consists of these materials, but it can additionally have been activated by precious metals or other electro-catalysts, such as, for example, Raney nickel or sulfur-containing nickel. Electrochemical cells for the alkali metal chloride electrolysis additionally contain a diaphragm or a cation exchanger membrane, which separates the anode space and cathode space from one another. Cation exchanger diaphragms, i.e. perfluorinated membrane containing sulfonic acid groups or carboxyl groups, are preferably used, if highly pure caustic soda is to be obtained. The membranes are cation-selective, that is to say they allow only the sodium ions to pass through in sodium chloride electrolysis, whereas the chloride ions remain in the anode space.
In practice, larger electrolyzers are assembled from such electrolysis cells which consist of the anode space with the anode, the cathode space with the cathode and the cation exchanger membrane, and these electrolyzers can consist of a multiplicity of individual cells. Monopolar or bipolar connection can be used for such electrolyzers. Bipolar connections are preferred, since very large cell units can be operated with these.
Difficulties arise, however, in the current transition from cell to cell. Because of the different materials of cathode space and anode space, the current being conducted in each case via the rear wall thereof, and above all because of the passivation of the titanium in an air atmosphere, large transition resistances and hence considerable voltage losses occur.